A carbonylation process of a methanol method (methanol method acetic acid process) is known as an industrial method for producing acetic acid. In this process, for example, methanol and carbon monoxide are reacted in the presence of a catalyst in a reaction vessel to produce acetic acid. The reaction mixture is evaporated in an evaporator, and the vapor phase is purified in a lower boiling point component removal column and subsequently in a dehydration column so that product acetic acid is prepared. Alternatively, product acetic acid is prepared via a higher boiling point component removal column subsequent to the dehydration column, and further, a product column.
In such an acetic acid production process, acetaldehyde generated by reduction of methyl iodide in the reaction system is converted into crotonaldehyde or 2-ethyl crotonaldehyde by aldol condensation, and thereby a potassium permanganate test value (permanganic acid time) of the product acetic acid is worsen. Moreover, after crotonaldehyde and acetaldehyde react, when hydrogen reduction and iodination is carried out, hexyl iodide is generated. If hexyl iodide is contained in the product acetic acid, a used palladium catalyst is deactivated in producing vinyl acetate using this acetic acid.
Conventionally, the two roughly classified methods have been adopted industrially in order to lower crotonaldehyde or 2-ethyl crotonaldehyde: (i) a method for suppressing generation of crotonaldehyde in a reaction system by removing acetaldehyde generated as a by-product in the reaction system from methyl iodide in the purification step and by lowering acetaldehyde in methyl iodide recycled to the reaction system; and (ii) a method of direct oxidative decomposition with ozone of crotonaldehyde contained in crude acetic acid obtained in the middle of a purification process (Patent Literatures 1 and 2). However, both separation and removal apparatus of acetaldehyde and ozonization apparatus are expensive. Until now, an improvement in the potassium permanganate test value of product acetic acid is extensively dependent on these methods to lead to an increase in apparatus cost. On the other hand, a method of treatment with a cation exchange resin substituted with a silver ion is known as a removal method of hexyl iodide. Meanwhile, in the treatment method using such a silver-substituted ion exchange resin, the silver-substituted ion exchange resin easily deteriorates to cause an increase in proportional cost. In National Publication of International Patent Application No. 2011-518880, a relationship of a flash temperature in case of introducing and heating reaction mixture of assumed composition in an evaporator with vapor and liquid composition discharged from a flasher is considered using a semiempirical simulator. However, this literature does not disclose or suggest that the acetaldehyde concentration and the methyl acetate concentration in the aqueous phase or the methyl acetate concentration in the organic phase of an overhead condensate obtained by subjecting vapor discharged from a flasher to a lower boiling point component removal column is controlled to decrease the concentrations of acetaldehyde, 2-ethyl crotonaldehyde and hexyl iodide in the reaction vessel.